1. Field of the Invention
The present invention relates to a measurement method for obtaining harmonic load-pull data for frequency multiplication by mechanically controlling the input/output impedances of transistors or diodes or other such devices having a frequency multiplication function and directly measuring the conditions at which output power, conversion efficiency and other such characteristics are optimal with respect to the target multiplication signal, and to an apparatus using the method.
2. Description of the Prior Art
In recent years there has been considerable research and development into high data rate Ka-band wireless communications systems. One of the issues is securing highly stabilized, low phase noise signal sources that are required for practical digital radio systems using, for example, BPSK, QPSK, or 16QAM Phase-locked oscillators (PLOs) are mentioned as a promising candidate From the standpoints of cost, power consumption, and phase noise, instead of direct phase locking of millimeter-wave oscillators, the use of a microwave PLO followed by one or multiple frequency multipliers is considered to be an effective approach. The operation of a frequency multiplier is based on device nonlinearity, and in most cases, multiplier design has been performed using large-signal device modeling on a microwave circuit simulator. The accuracy of the device modeling, however, has often been insufficient with respect to class B operation, which is extensively used as the operating point of frequency multipliers.
Load-pull (source-pull with respect to the signal source) is known as an alternative approach to device modeling. This method consists in connecting a tuner to the input or output of the device terminal, or a pair of tuners to the input and output of the device terms respectively, and adjusting the tuner impedance(s) while measuring them to find directly the optimum impedance(s) in terms of output power, gain, and so forth. Load-pull has been employed mainly for characterizing high power devices and in the designing of high power amplifiers. Mechanical tuners are used for performing relatively straightforward load-pull measurements, and more recently, automatic tuners are commercially available that enable accurate measurements in a short time.
FIG. 13 shows a conventional load-pull measurement system using mechanical tuners. In load-pull measurement system 100, a pair of mechanical tuners are connected to source measurement system 102 and load measurement system 103 of the device under test (DUT) 101, respectively. The impedances are mechanically varied at the target frequency to establish the conditions under which, for actual input/output signal levels, impedance matching is performed, to optimize output power and gain, etc.
FIG. 14 shows a configuration of a typical coaxial mechanical tuner, called a slug tuner. This is configured as a slabline, with a center conductor 106 arranged at a central position between a pair of parallel, opposed ground planes 105a and 105b. FIG. 14(a) shows a cross section perpendicular to the slabline, and FIG. 14(b) shows a cross section parallel to the slabline. With a metallic slug 107 inserted down into the slabline, arbitrary impedances are generated by adjusting the position of the slug 107 horizontally and vertically with respect to the slabline. Signals are input to the tuner via an input coaxial terminal 108a, and are output via an output coaxial terminal 108b. 
The electrical angle from the output terminal of the DUT 101 is varied by adjusting the horizontal distance from the input terminal 108a to the slug 107. The absolute value of the reflection coefficient corresponding to the load impedance observed from the output terminal of the DUT 101 is varied by adjusting the vertical distance from the center conductor 106 to the slug 107. A short-circuit condition (reflection coefficient absolute value of 1) is effected by bringing the slug 107 into proximity of the center conductor 106; conversely, by increasing the distance between the slug 107 and the center conductor 106, it is possible to minimize the effect on the electromagnetic field of the transmission line, enabling an impedance of 50 ohms (reflection coefficient absolute value of 0) Since a DUT cannot normally be accessed directly from a coaxial component, a transforming structure is necessary. There are a number of access means, with the transforming structures being referred to as test fixtures.
When a conventional mechanical tuner with one slug is used for measuring harmonic load-pull for frequency multiplication, it is possible to find the load impedance at the target harmonic frequency for which parameters such as multiplication output power are at op levels by carrying out measurements while varying the impedance of the output tuner at the target multiplication frequency. However, with respect to improving the frequency multiplier performances such as conversion gain, not only does the load impedance at the target harmonic frequency ZL (nF0) (where n is the multiplication order) of the output network have to be set, but also the fundamental load impedance ZL (F0) that satisfies the short-circuit condition with a specific electrical angle. That is, although it is known that the performance is improved by optimizing the xcex81 in ZL (F0)=j50 xcexa9tan xcex81, in a conventional mechanical tuner, as described above, a short-circuit condition can be realized by setting the slug near the center conductor, so a short circuit with respect to the fundamental signal also formed a short circuit with respect to the multiplication signal, making it impossible to optimize the load impedance at the target harmonic frequency. Moreover, when a given load impedance at the target harmonic frequency is realized by setting the position of the slug, the fundamental load impedance is set at a specific value that is dependent on the slug position setting. Thus, with a prior art mechanical tuner having one slug, it is not possible to independently control the fundamental and harmonic load impedances.
Moreover, frequency multiplier performance is considered to be dependent not only on the fundamental source impedance ZS (F0) of the input circuit, but also on the source impedance at the multiplied frequency. As for frequency doublers, it has been reported that performance is also dependent on the second harmonic source impedance ZS (2F0) that satisfies the short circuit condition with a specific electrical angle, that is, the xcex82 in ZS (F0)=j50 xcexa9tan xcex82. That is, it is also desirable to be able to independently control the fundamental and harmonic load impedances in the source mechanical tuner, which is not possible in the case of the single-slug mechanical tuner of the prior art described above.
In order to be able to measure the performance limits of frequency multiplication devices, an object of the present invention is to provide a method for measuring harmonic load-pull for frequency multiplication that, with the fundamental load impedance and the source impedance at the target harmonic frequency each set at an optimal state, enables the target harmonic load impedance and the fundamental source impedance to be individually controlled, respectively, and to provide an apparatus for measuring harmonic load-pull for frequency multiplication using the method.
To attain the above object, the present invention provides a method for measuring harmonic load-pull for frequency multiplication to obtain a load impedance and a source impedance for which frequency multiplication performance of a frequency multiplication device is optimized, the method comprising;
supplying a fundamental frequency signal to a frequency multiplication device under test (DUT) from a source measurement system that includes a source mechanical tuner for adjusting a fundamental source impedance of an input signal, and obtaining a load impedance at which multiplication performance of the frequency multiplication device is optimum from a load measurement system that includes a load mechanical tuner for adjusting a load impedance of a target even-order multiplied signal among harmonics included in signal output of the frequency multiplication device;
independently controlling the fundamental load impedance and an even-harmonic load impedance by means of a load mechanical tuner that includes a control means that functions as an open-ended stub that is one-quarter wavelength long at the fundamental frequency and which at that position satisfies a short-circuit condition with respect to the fundamental signal, for controlling a fundamental load impedance that in accordance with a position at which the stub is set determines an electrical angle from an output section of a device under test to a short-circuit point;
independently controlling fundamental source impedance and even-harmonic source impedance by means of a source mechanical tuner that includes control means that functions as a short-circuit stub that is one-quarter wavelength long at the fundamental frequency, meaning a half wavelength long at the second harmonic frequency, and which at that position satisfies a short-circuit condition with respect to even harmonics that include a second harmonic signal, for controlling an even-harmonic source impedance that in accordance with a position at which the stub is set determines an electrical angle from an input section of a device under test to the short-circuit point.
The present invention also provides an apparatus for measuring harmonic load-pull for frequency multiplication to obtain a load impedance and a source impedance for which frequency multiplication performance of a frequency multiplication device is optimized, the apparatus comprising:
a source measuring system that supplies a fundamental frequency signal to a DUT from a source measurement system that includes a source mechanical tuner for adjusting a fundamental source impedance of an input signal, and a load measurement system that includes a load mechanical tuner for adjusting a load impedance of a target even-order harmonic signal within a harmonic included in signal output of the frequency multiplication device;
the load mechanical tuner of the load measuring system including control means that functions as an open ended stub that is a quarter wavelength long at the fundamental frequency and which at that position satisfies a short-circuit condition with respect to the fundamental signal and in accordance with a position at which the stub is set determines an electrical angle from an output section of a DUT to a short-circuit point;
the source mechanical tuner of the source measuring system including control means that functions as a short-circuit stub that is a quarter wavelength long at the fundamental frequency, meaning a half wavelength long at the second harmonic frequency, and which at that position satisfies a short-circuit condition with respect to even-harmonic signals that include a second harmonic, for controlling an even-harmonic source impedance that in accordance with a position at which the stub is set determines an electrical angle from an input section of a DUT to a short-circuit point;
the load mechanical tuner being used to independently control the fundamental load impedance and an even-harmonic load impedance, and the source mechanical tuner being used to independently control the fundamental source impedance and an even-harmonic source impedance.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.